1. Field of the Invention
The subject matter described herein relates generally to borescopes and endoscopes, and more particularly, to a borescope/endoscope which uses phase-shift analysis to provide 3D surface mapping and dimensional measurement.
2. Related Art
Borescopes and endoscopes are typically used for inspection inside a remote cavity, Most borescopes/endoscopes, referred to herein as a probes, employ an external light source coupled to fiber optic bundles in the probe to provide illumination of a remote object or surface at the distal end. When the object is illuminated, an internal image is formed by a lens system on an image sensor, and the image is relayed to a connected display, such as a video screen. The image sensor may be located at the proximal end of the probe, as with an optical rigid borescope or fiberscope, or at the distal end as with a video borescope or endoscope. Such systems are often used to inspect in inaccessible locations for damage or wear or to verify that parts have been properly manufactured or assembled. Among other things, it is desirable to obtain dimensional measurements to verify that damage or wear does not exceed an operational limit or that a manufactured part or assembly meets its specifications. It may also be desirable to produce a 3D model or surface map for comparison to a reference, 3D viewing, reverse engineering, or detailed surface analysis.
The image shown on the connected display varies in magnification and apparent size depending upon the distance between the object and the distal end of the probe. This leads to difficulties in directly determining dimensional measurements, especially in three spatial dimensions.
There are a number of known approaches for providing 3D data through a probe including splitting the view to gain a stereo image (stereo viewing), projecting a coarse pattern of dots onto the remote object, or using a single line to obtain a single image profile. Stereo methods can be used to create a 3D view, but can only provide information where two points on the image can be correlated. This can be problematic when little surface detail exists. The correlation process can also require significant processing, so producing a full 3D surface map can be time consuming. It is more typical to only correlate a small number of points needed for basic measurements. Projecting a course pattern of dots permits measurement to be obtained at the points of the dots. However, the areas between the dots are left to be interpolated, so any surface variations between them are lost. Finally, a single line profile provides useful information along that single profile, but proper positioning of the single line on the object of interest can be difficult, and measurements that require non co-linear points, such as point to line or area measurements, are subject to error if the surface is not flat or the view is not perpendicular to the surface. The scanning of a single profile line that is often employed in commercial systems to build a 3D surface map is generally not practical in a small, probe due to size constraints.
Other limitations also exist regarding the approaches discussed above. For example, a large computing capacity is often required to implement the solutions, and highly skilled technicians are needed to operate the equipment. In addition, the above approaches may not be appropriate when a dense 3D full surface map or full-field object measurement is desired. Without the full-field data, imperfections on a surface or object may be missed entirely. Thus, it is desirable to provide a probe that offers full-field surface mapping.
Full-field object data can be obtained through phase-shifting. Phase-shifting is an analysis technique used for non-contact optical metrology applications. Phase-shifting typically involves projecting one or more sets of parallel lines that cross the field of view (FOV) of a camera. As the object distance changes, the parallel lines, or fringe sets, shift across the FOV. Which line is which, or absolute phase, must be determined in order to make accurate measurements and obtain an accurate surface map. The absolute phase at a given point in the image is defined as the total phase difference (2π times the number of line periods) between a reference point in the projected line pattern and the given point. The reference point can be arbitrarily defined.
There are a number of known approaches to decipher which line is which and determine absolute phase. Some approaches include employing multiple fringe sets with physical horizontal offsets resulting in a relative phase that changes with distance or using multiple fringe sets with physical axial offsets to change the period with distance. Most techniques use additional projections. For example, to assist in determining the absolute phase, an extra line may be projected to give a starting reference point. The determined absolute phase combined with the fringe set position in the FOV are commonly used to determine absolute object distance.
Phase-shifting methods have not been practical for use in devices such as borescopes and endoscopes. The equipment required to project suitable line patterns for phase-shifting methods usually include a projector, scanner, piezo mirror, or similar item. Among other things, the size limitations of probes make the use of typical equipment mechanically challenging.
Thus, it is desirable to provide a practical mechanical configuration of a probe that is able to perform measurements and 3D surface mapping based on phase-shift analysis. It is further desirable to provide an analysis technique appropriate for the configuration that assists with fringe ordering to determine absolute phase and object distance even when complex surfaces are involved.